Substantial effort and expense have been devoted to the development of plain paper compatible direct marking technologies. The research and development activities relating to drop on demand and continuous stream ink jet printing account for a significant portion of this investment, even though conventional ink jets suffer from the fundamental disadvantage of requiring nozzles with small ejection orifices which easily clog. Unfortunately, the size of the ejection orifice is a critical design parameter of an ink jet because it determines the size of the droplets of ink that the jet ejects. As a result, the size of the ejection orifice cannot be increased, without sacrificing resolution.
Acoustic printing is a potenially important, alternative direct marking technology. It is still in an early stage of development, but the available evidence indicates that it is likely to compare favorably with conventional ink jet systems for printing either on plain paper or on specialized recording media, while providing significant advantages on its own merits. More particularly, acoustic printing has increased intrinsic reliability because there are no nozzles to clog. As will be appreciated, the elimination of the clogged nozzle failure mode is especially relevant to the reliability of large arrays of ink ejectors, such as page width arrays comprising several thousand separate ejectors. Furthermore, small ejection orifices are avoided, so acoustic printing can be performed with a greater variety of inks than conventional ink jet printing, including inks having higher viscosities and inks containing pigments and other particulate components. In keeping with still another feature of the technology, a copending and commonly assigned United States patent application of Elrod et al, which was filed Dec. 19, 1986 under Ser. No. 944,286 on "Variable Spot Size Acoustic Printing" shows that the size of the individual picture elements ("pixels") printed by an acoustic printer may be controlled during operation, either by varying the size of the individual droplets that are ejected, or by regulating the number of droplets that are used to form the individual pixels of the printed image.
As is known, an acoustic beam exerts a radiation pressure against objects upon which it impinges. Consequently, if an acoustic beam impinges on a free surface (i.e., liquid/air interface) of a pool of liquid from beneath, the radiation pressure which the beam exerts against the free surface may reach a sufficiently high level to release individual droplets of liquid from the surface of the pool, despite the restraining force of surface tension. To accomplish that, the acoustic beam advantageously is brought to focus on or near the surface of the pool, thereby intensifying its radiation pressure for a given amount of input power. These principles have been applied to ink jet and acoustic printing previously, using ultrasonic (rf) acoustic beams to release small droplets of ink from pools of ink. For example, K. A. Krause, "Focusing Ink Jet Head," IBM Technical Disclosure Bulletin, Vol 16, No. 4 September 1973, pp. 1168-1170 describes an ink jet in which an acoustic beam emanating from a concave surface and confined by a conical aperture is used to propel ink droplets out through a small ejection orifice. Lovelady et al. U.S. Pat. No. 4,308,547, which issued Dec. 29, 1981 on a "Liquid Droplet Emitter" showed that the small ejection orifice of the conventional ink jet is unnecessary. To that end, they provided spherical piezoelectric shells as transducers for supplying focused acoustic beams to eject droplets of ink from the free surface of a pool of ink. They also proposed acoustic horns driven by planar transducers to eject droplets of ink from an ink coated belt. Thereafter, to reduce the cost of acoustic printheads and to simplify the fabrication of multiple ejector arrays, a copending and commonly assigned U.S. Pat. No. 4,697,195 of C. F. Quate et al., which issued Sept. 29, 1987 on Nozzleless Liquid Droplet Ejectors introduced a planar interdigitated transducer (IDT) and planar IDT arrays, Quate et al also disclosed that the droplet ejection process can be controlled, either directly by modulating the acoustic beam or indirectly in response to supplemental bursts of power from a suitably controlled rf source.
The IDT provides an economical technology for fabricating arrays of acoustic droplet ejectors, but is hollow beam focal pattern results in a higher sensitivity to minor variations in the surface level of the ink than is desired for some applications. Accordingly, there still is a need for a technology which permits arrays of high ejection stability acoustic droplet ejectors to be assembled at moderate cost.